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Have a look at the installation instructions at the Flask website. The most important steps are listed here.
On MacOS X or GNU/Linux, run one of the following:
$ sudo pip install virtualenv
or
$ sudo easy_install virtualenv
On Ubuntu, you can use
$ sudo apt-get install python-virtualenv
Get and activate the latest flask version:
$ git clone http://github.com/mitsuhiko/flask.git
Cloning into 'flask'...
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$ cd flask
$ virtualenv venv --distribute
New python executable in venv/bin/python
Installing distribute............done.
$ . venv/bin/activate
$ python setup.py develop
...
Finished processing dependencies for Flask
Install Pillow
$ pip install Pillow
Download and extract fsikasse or clone the git repository.
To initialize the database, run in the fsikasse-directory
$ flask --app=fsikasse initdb
And start the app with
$ flask --app=fsikasse run
From now on, it is sufficient to run in the flask-directory
$ . venv/bin/activate
and then start the app in the fsikasse-directory with
$ flask --app=fsikasse run